The present invention provides a method and apparatus for drilling a borehole with an optionally rectilinear or arcuate center line into earth formations.
Tools of this type, which are used for navigational drilling without tool change, are known to be available in various designs.
In order to create an angle of deflection--which at the same time determines the build-up rate to be achieved--for the rotation axis of the drill bit shaft during directional drilling, the first and the second stabilizer of a first well-known tool (U.S. Pat. No. 4,465,147) are arranged eccentrically on the casing--which has the shape of a straight tube--of the rotary drilling tool. In directional drilling operations, such a design imparts a deflection, which determines the angle of deflection, to the casing.
In a second well-known tool (U.S. Pat. No. 4,739,842), the stabilizers are concentrically arranged on the casing of the rotary drilling tool, and the casing is provided with sections deflected relative to the principal axis of the tool, which define two bends which face in opposite directions and which in combination with each other determine the angle of deflection. According to a further development of this tool, as also disclosed in the aforementioned U.S. Pat. No. 4,739,842, the deflection of the casing regions can be designed in such a way that only one single bend between the two stabilizers determines the angle of deflection.
Instead of one or two bends in the region of the casing between the first and the second stabilizer, a third well-known tool of the type mentioned in the introduction provides for a bend between the rotary drilling bit and the first stabilizer (U.S. Pat. No. 4,492,276). This bend is formed in such a way that the bit shaft is carried in the lower area of the casing--which has the form of a straight tube--at an angle relative to the axis of this casing and exits at a slant from the end of the casing.
In a fourth well-known tool (U.S. Pat. No. 4,485,879), the bit shaft is carried in the casing of the rotary drilling tool, with its rotation axis being laterally and parallelly offset with respect to the axis of the casing.
The present invention provides a method and apparatus which has a higher accuracy of tracking and a higher penetration rate during directional drilling while at the same time reducing its wear.